1. Technical Field
The present invention relates to a color image processing to technology for a color output device. More particularly, the invention relates to a method and an apparatus for adjusting a white point with higher accuracy in a liquid crystal display device.
2. Prior Art
As display devices for image displaying on a personal computer, a television set or the like, and for various other monitors, in addition to a CRT, liquid crystal devices (LCD) have come into wide use in recent years. In a color display system using the CRT, the LCD or the like, it is considered ideal to bring colors to be reproduced as close as possible to natural ones. It is also required that an apparatus should make an automatic adjustment or an operator (user) should make a manual adjustment according to the installing state of the apparatus using the CRT or the LCD, i.e., an environment of illumination or the like where the apparatus is set, in order to display an optimal color suited to each environment. In addition, it is strongly demanded that the capability of displaying a same color irrespective of the kind of an output device should be provided. Among these technologies, great importance is placed especially on a white point adjustment designed to adjust an achromatic color level in displaying, and such a white point adjustment has conventionally been realized for a color monitor or the like.
To treat all natural colors in a quantitative manner, a ClExy chromaticity diagram shown in FIG. 8 is available. This drawing represents a hue and color saturation of a given color on the basis of the position of a chromaticity coordinate, specifically showing a chromaticity coordinate represented by the axis of abscissa x=X÷(X+Y+Z) and the axis of ordinate y=Y÷(X+Y+Z) in tristimulus values X, Y and Z of an XYZ display system. For a portion on a closed curve C formed in a horseshoe shape and an inner portion thereof in the drawing, the entire range of colors seen by human eyes is _shown. Points R. G and B in the drawing respectively represent display colors based only on primary colors of R (red), G (green) and B (blue) in a particular color display system. All the colors on the sides of a triangle RGB and in an inner portion thereof can be expressed by means of proper mixing of R, G and B. Further, white having maximum luminance can be obtained typically as a mixed color W when each of R, G and B is set at maximum luminance, and this white color is usually located in the vicinity of an intersection of medians of the triangle R, G and B as shown in the drawing.
When designing a color display system, a more optimal white point is decided by adjusting maximum luminance values of the points R, G and B or changing the positions of the points R, G and B in the drawing. For example, in the color display system using the LCD, preferably, a white point should be decided by taking into consideration a spectral radiation characteristic of a backlight or a transmission characteristic of a color, filter.
In the prior art, for example, there is Japanese Patent Laid-Open No. Hei 2(1990)-271389 gazette. This gazette discloses a technology to correct gray level data so as to set a liquid crystal luminance-gray level data characteristic to be linear, in order to enable full-color image displaying having excellent display quality to be performed by preventing color shifting. Another gazette of Japanese Patent Laid-Open No. Hei 2(1990)-271793 discloses a technology to adjust chromaticity by uniformly increasing luminance of a low gray level side of B (blue) or R (red)/G (green) and preventing a reduction in luminance of the entire screen, when low gray level displaying continues.
On the other hand, as one of the problems inherent in a TFT LCD monitor or the like, a phenomenon of blue shifting occurs in halftone gray (halftone achromatic color) especially at a low gray level. This phenomenon specifically refers to a case where during displaying of an achromatic color (i.e., color with R, G and B set at the same gray level) on the TFT LCD device, the color becomes bluish (i.e., the chromaticity coordinate shifts toward a blue color) as a gray level value thereof is, reduced.
FIG. 9 shows a color temperature change for each gray level in the LCD by using a CIE chromaticity coordinate. The axis of abscissa x and the axis of ordinate y form a chromaticity coordinate, which is expressed by an abscissa x=X÷(X+Y+Z) and an ordinate y=Y÷(X+Y+Z) in tristimulus values X, Y and Z. In the drawing, a broken line indicates a black body locus, showing that a color becomes bluish with a color temperature increased toward the oblique left lower direction. In the drawing, gray to levels are also shown that, in the LCD panel, from the highest (255) to the lowest levels (0) respectively by 5 points when viewed from front face and when viewing angles are increased by means of shifting of 15 degrees, 30 degrees, 45 degrees and 60 degrees in a horizontal direction. The moving direction of a white point is indicated by a solid-line arrow A when a gray level is decreased, and the moving direction of each of the gray to levels when viewed from the front face and when the viewing angles are increased by means of shifting of 15 degrees, 30 degrees, 45 degrees and 60 degrees towards the front face and in the horizontal direction is indicated by a broken-line arrow B.
As apparent from FIG. 9, it can be understood that as a characteristic of the LCD, the white point defined bye the highest gray level is greatly shifted at the other halftone gray levels. In other words, toward the low gray level, the white point is shifted to a bluish direction on the CIE chromaticity coordinate. This phenomenon is caused by the change of light leakage, which occurs depending on the inclination of a liquid crystal when the liquid crystal cuts off a light. Once such a phenomenon occurs, the white point is greatly shifted from its setting at the low gray level even if the white point of the highest gray level can be adjusted to a desired chromaticity coordinate (color temperature). This phenomenon has been very conspicuous in certain kinds of LCD panels, posing a new problem to be solved.
As shown in the drawing, because of color shifting caused by a viewing angle, in connection with the foregoing phenomenon of color shifting at the halftone gray level, color shifting is increased from a white point spec value of a white color at the halftone gray level. There has been a strong demand for assurance of a high viewing angle in the LCD in recent years. But a more conspicuous occurrence of color shifting as the angle of viewing (viewing angle) the display is inclined from the front face has been another serious problem.
In the gazettes of Japanese Patent Laid-Open No. Hei 2(1990)-271389 and Patent Laid-Open No. Hei 2(1990)-271793 of the prior art, no mention is made for the need to correct white point shifting at the halftone gray level. Especially, in the gazette of Japanese Patent Laid-Open No. Hei 2(1990)-271389, a technology is disclosed that a luminance ratio of R, G and B is maintained constant at all the gray levels. But this maintenance technology of the constant luminance ratio is completely different from maintenance technology of a constant white point at all the gray levels in the case of the LCD.
Furthermore, for example, even with the assumption that setting of a color temperature is changed by a method: of changing luminance and mixture of R, G and B colors or a method of adjusting each luminance of a plurality of fluorescent tubes having different spectrum characteristics, panel luminance varies between high and low temperature sides in the case of adjusting a white point defined by a highest gray level. In other words, a problem has been occurred that a highest luminance defined at a certain white point cannot be guaranteed at other white points.